On p.56 of Bruce Donaldson's German, An Essential Grammar we read:
When ‘it’ and ‘them’ with reference to things, as opposed to people, are preceded by a preposition, the above pronominal forms cannot be used; in other words in es, auf es etc. are not possible at all, and in ihn, auf ihm etc. and in sie, auf ihr etc. are only possible if referring to a person. German uses the so-called prepositional adverb in such cases, formed by combining da (there, an adverb) with the preposition in question, e.g. damit (with it/them), dahinter (behind it/them).
Does this mean that, relative pronouns when they refer to "things" and not people, should be replaced with forms with "da-"? For instance, in this sentence:
Die Stadt, in der er jahrelang als Kind gewohnt hat, hat er nie wieder besucht.
is it correct to replace in der with darin? In particular is it possible to use "da(r) + preposition" forms in place of "preposition + relative pronoun" forms (such as the aforementioned in der)?
I have an answer to these questions; but I am not sure whether it is correct or not. I think what Bruce Donaldson says is limited to third personal pronouns; not relative pronouns (even though the relative pronoun is referring to a "thing" in the main clause. And, I think "da(r) + preposition" forms cannot be used as relative pronouns; "wo(r) + preposition" forms (such as worin) should be utilized. I am not sure I am correct.